1. Field of the Invention
This invention relates to organosiloxane compositions that cure by a platinum-catalyzed hydrosilation reaction. More particularly this invention relates to organosiloxane compositions exhibiting a desirable combination of reduced flammability and adhesion to both organic polymers and aluminum substrates following curing.
2. Background Information
The curing characteristics of organosiloxane compositions that cure by a platinum-catalyzed hydrosilation reaction and the properties of the cured materials obtained from these compositions are desirable for a variety of end use applications, particularly in the electrical and electronics areas. The cured materials are useful as protective coatings, potting materials and encapsulants for integrated circuits and other electronic devices and as coatings for printed circuit boards.
A shortcoming of organosiloxane compositions that cure by a hydrosilation reaction is their inability to adhere strongly to many substrates, particularly plastics and certain metals. One method for remedying this deficiency is by the use of primer compositions or adhesion promoting additives containing silanes and organosiloxanes with a plurality of silicon-bonded hydrolyzable groups and at least one organofunctional substituent that is bonded to silicon through at least one carbon atom. Primer compositions are applied to substrates to which adhesion is desired prior to application of the curable organosiloxane composition. Adhesion promoters are present in the curable organosiloxane composition.
Examples of hydrolyzable groups that are present in known adhesion-promoting organosilicon compounds are alkoxy, carboxy such as acetoxy, ketoximo such as methylethyl ketoximo, and amido. The organofunctional group is typically one that will react with the substrate to which adhesion is desired. Examples of suitable organofunction groups include but are not limited to alkenyl such as vinyl and allyl, aminoalkyl, epoxyalkyl, mercaptoalkyl and (meth)acryloxyalkyl.
U.S. Pat. No. 4,808,640 describes organic or silicone resin compositions containing up to 100 weight percent, based on the weight of the resin, of particles obtained by curing an organosiloxane composition containing an alkoxy substituted silane such as 3-glycidoxypropyl-trimethoxysilane and an aluminum or zirconium compound.
U.S. Pat. No. 5,166,293 describes organosiloxane compositions containing a polyorganosiloxane with at least two vinyl groups per molecule, organohydrogensiloxanes, a platinum-containing hydrosilation catalyst, and organosiloxanes containing alkoxy and epoxy groups.
The use of organic or organosilicon compounds containing epoxy groups in combination with an organic aluminum compound as an adhesion promoter for organosiloxane compositions that cure by a platinum catalyzed hydrosilation reaction is taught in Japanese patent no. 88-48902.
The present inventors discovered that addition of even small amounts of organoaluminum compounds such as aluminum acetylacetonate substantially increases the flammability of the composition.
The use of hydrated aluminum oxide or magnesium oxide in combination with platinum to reduce the flammability of elastomers prepared from peroxide-curable organosiloxane compositions is described in U.S. Pat. No. 4,288,360.
A disadvantage of inorganic flame retarding agents such as hydrated aluminum oxide and magnesium hydroxide is their adverse effect at moderate to high filler loadings on the adhesion of organosiloxane compositions that cure by a platinum catalyzed hydrosilation reaction.
The present inventors have found that when used at concentrations above about 10 percent, based on the total weight of the curable composition, these flame retarding agents may at least partially offset the improvement in adhesion obtained by the addition of an organic epoxide compound, a chelated organoaluminum compound and an alkoxy silane or condensation product of the silane. The present inventors also discovered that the organic aluminum compounds that have been used to promote the reaction of epoxide groups substantially increase the flammability of cured elastomeric products prepared from curable organosiloxane compositions containing these compounds as an component of the adhesion-promoting additive.
One objective of this invention is to provide a combination of additives for organosiloxane compositions that provide both reduced flammability and adhesion without adversely affecting the ability of the compositions to cure by a platinum group metal catalyzed hydrosilation reaction.